Colloquium Ehrenfestii: L.W. Molenkamp:"HgTe as a Topological Insulator "





Oortbuilding, De Sitterzaal, Leiden


Colloquium Ehrenfestii

Wednesday 20 November 2013

L.W. Molenkamp Physics Institute (EP3), Würzburg University

Title: HgTe as a Topological Insulator 

Abstract: HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is a semimetal, lowering the crystalline symmetry opens up a gap, turning the compound into a topological insulator.

The most straightforward way to do so is by growing a quantum well with (Hg,Cd)Te barriers.  Such structures exhibit the quantum spin Hall effect, where a pair of spin polarized helical edge channels develops when the bulk of the material is insulating.

Our transport data[1-3] provide very direct evidence for the existence of this third quantum Hall effect, which now is seen as the prime manifestation of a 2-dimensional topological insulator.

To turn the material into a 3-dimensional topological insulator, we utilize growth induced strain in relatively thick (ca. 100 nm) HgTe epitaxial layers. The high electronic quality of such layers allows a direct observation of the quantum Hall effect of the 2-dimensional topological surface states[4]. These states appear to be decoupled from the bulk. This allows us to induce a supercurrent is induced in the surface states by contacting these structures with Nb electrodes[5].

[1] M. König et al., Science 318, 766 (2007).

[2] A. Roth et al., Science 325, 294 (2009).

[3] C. Brüne et al., Nature Physics  8, 486 (2012).

[4] C. Brüne et al., Phys. Rev. Lett. 106, 126803 (2011).

[5] L. Maier et al, Phys. Rev. Lett. 109, 186806 (2012);

[6] J.B. Oostinga et al., PRX 3, 021007 (2013).

Please Note:  Before the Colloquium, there is a common dinner in the canteen located on the ground floor of the Oort building. This dinner starts at 18:00 hours sharp and is free of charge, under the condition that one attends the colloquium and that one has made a reservation before noon on the Tuesday preceding the colloquium. The registration form is only accessible from within the University (to block spammers); if you would like to register for the dinner from outside the University, please send an email to

The revolving doors of the Oort building, alongside the Kamerlingh Onnes Laboratory, are accessible until 9 pm. After that time, one should instead use the sliding doors directly adjacent to the Huygens building: these can be opened with an electronic key.